There are conventionally known techniques for decomposing water into hydrogen and oxygen by irradiating a semiconductor material serving as an optical semiconductor with light (see, for example, Patent Literature 1). Patent Literature 1 discloses a technique in which an n-type semiconductor electrode and a counter electrode are disposed in an electrolyte and the surface of the n-type semiconductor electrode is irradiated with light to obtain hydrogen and oxygen from the surfaces of these electrodes. Specifically, the use of a TiO2 electrode or the like as the n-type semiconductor electrode is described therein.
However, the sunlight utilization efficiency of the n-type semiconductor electrode disclosed in Patent Literature 1 is still not high enough. For example, since an anatase type TiO2 electrode has a band gap of 380 nm, only about 1% of sunlight can be utilized.
In order to solve the above problem, it is proposed, for example, to use optical semiconductor materials with smaller band gaps. For example, Patent Literature 2 proposes the use of a NbON optical semiconductor obtained by calcining Nb2O5 in a high-temperature ammonia atmosphere. Since the NbON optical semiconductor has a smaller band gap of about 600 nm, the sunlight utilization efficiency can be increased. Patent Literature 3 discloses an electrode (electrode catalyst) containing a NbON optical semiconductor attached thereto. This electrode can also be used for photolysis of water.